This is truly a golden age for professional tennis. With
Rafael Nadal roaring back to the top of the game after a seven-month injury
hiatus, Novak Djokovic extending his major semifinal streak to 13, Andy Murray
securing his first Wimbledon championship and Roger Federer trying out a larger
racquet frame to prolong his career, the debate over who is the greatest tennis
player of all time has taken on a powerful momentum.

The women, however, never get to be a part of these
discussions.

There's a very easy explanation for this: when it comes to
the women, there's nothing to debate.

Serena Williams, at 31, is the best player on the WTA tour
by far. She's won 16 major singles titles. Her biggest "rivals" at the moment,
Victoria Azarenka and Maria Sharapova, have a combined 3-25 record against her.
It has become conventional wisdom that she is the best woman to ever pick up a
fuzzy yellow ball.

But is it so cut and dried?

The case can be made for Serena as the best ever, and it has
been, but it's actually still a rather tough sell.

In 2003, when the American had the "Serena Slam" in her
pocket and was going for the real Grand Slam, the diminutive Justine Henin beat
her at the French Open. That match is best remembered for Henin's questionable
sportsmanship when she refused to acknowledge that she had disrupted Serena's
service motion by raising her hand. But it's difficult to argue that Serena
would have won if only Henin had admitted to her etiquette transgression. The
fact is, Serena was at her very best -- and it wasn't enough to beat Henin. The
little Belgian would prove four years later that she didn't need gamesmanship
to be the best in the game. In 2007, she topped Serena in succession at the
French Open, Wimbledon and the U.S. Open, slamming the door shut -- or so we
thought at the time -- on the Serena era.

Federer's critics, we all know, insist that the 17-time
major champion can't be the greatest ever for the simple reason that he's had
such a hard time beating Nadal. Similarly, every Serena antagonist out there
can argue that Serena can't be the greatest ever when the smaller, less
powerful, less athletic Henin could top her even on grass and hard court.
(Henin retired a couple of years ago when both her body and her will broke down
on her.)

Serena might be aware of this line of thinking. She had an
epiphany of sorts when she was sidelined for months in 2010-11 with various
significant injuries. She saw the sands falling through the hourglass and
decided she would finally commit herself fully to tennis. The best rebuttal to
the Henin interregnum (as well as the Kim Clijsters, Jelena Jankovic, Dinara
Safina and Azarenka rankings boomlets) is that Serena has always had trouble
maintaining interest in playing tennis. She has periodically drifted away from
the tour to pursue business, publishing, reality-TV and other interests.

But of course it's not as simple as that. She lost focus for
a very specific reason: she had vanquished her older sister, seven-time major
champion Venus Williams. This had to be both thrilling and devastating for her.
Throughout childhood, Serena's whole life had been defined in relation to Venus,
and until 2002 it almost always had been found wanting. Venus, after all, was
older and so was faster and stronger and more mature.

The untold story of Serena's success on court is its relationship
to Venus' failure. For years, Venus was the anointed one; her father declared
that she would carry the family out of L.A.'s tough streets and into legend. And
she did it. In 2001, Jon Wertheim wrote a bestselling book about the WTA tour
called "Venus Envy." The older sister was the best in the world, and many
observers thought she would end up the best of all time.

But, because of Serena, Venus was the best only for a very
short period of time. Once she reached her full powers, Serena proved
physically stronger and mentally tougher than her beloved sister.

View full sizeSerena Williams after winning the 2013 French Open.The Associated Press

Venus rapidly faded from the scene after Serena reached the
heights, only occasionally rising back up to meet her potential. Now struggling with a fatigue-causing condition, she's been an
also-ran for almost a decade, kept from being viewed as a tennis
anachronism only by her symbiotic connection to her sister. Venus'
over-the-top, pogo-stick celebration after defeating Lindsay Davenport to win
the 2005 Wimbledon crown, her first major singles titles in four years,
suggested that she had already gone through the mourning process of losing
her place in the world and had accepted her thwarted expectations. She had
learned the hard way that one step off the highway of success can lead to ruin. (Remember how she
celebrated with her sister and snapped her picture on court when Serena beat her
in the 2002 French Open final to start Serena's run?) The
Wimbledon victory over Davenport truly was a joyous shock to her, a
mind-blowing blast from the past.

Serena took their change in status almost as hard. Once she
had defeated Venus in four straight major finals in 2002-03, she believed --
consciously or not -- that there were no more tennis mountains to climb. She was determined to be the best in the world, and that meant being better than Venus. Beating
Henin or Clijsters or Davenport just wasn't the same. It took her years to
realize that sustained success on court could still matter in the post-Venus era
that she had brought about.

Forced to sit and think while healing in 2011, Serena
realized that the game will indeed go on without the Williams sisters. And, at
the same time, that no matter what she does with the rest of her life, she will
be remembered first and foremost for her tennis. And so she decided she would close
her career by giving the sport her all.

The results have been impressive. Last year Serena won
Wimbledon, Olympic gold and the U.S. Open. This season she put together a 34-match
win streak and won the French Open, putting her eight shy of Margaret Smith
Court's all-time record for major singles championships. She has proved once
again that when she's firing on all cylinders, no one can beat her.

And yet, this dominance is beginning to look much broader
than it is deep. Though fit and fully focused, Serena has won just one of the
three major tournaments played in 2013. She lost to an unseeded teenager at the Australian Open. At Wimbledon, where her game is most lethal,
she fell to the 23rd seed. Age finally might be making a fact of what
had long been a choice: that Serena shines very, very brightly -- but not all
the time.

This surely is frustrating for her, now that she's committed to year-round excellence. But she should enjoy it while it lasts.